Polycarbonate polymers are excellent molding materials because products made therefrom have high impact strength, toughness, high transparency, wide temperature limits (high impact resistance below -60.degree. C. and a UL thermal endurance rating of 115.degree. C. with impact), good dimensional stability, high creep resistance and electrical properties which qualify it as sole support for current carrying parts.
Polycarbonates are, however, very difficult to fabricate from melts for the reason that melts thereof have exceptionally high viscosities. Attempts to overcome this difficulty by the incorporation with the polycarbonate of materials known to reduce the viscosity of other resins have very generally been unsuccessful. Many standard viscosity control agents appear to have little or no effect on the viscosity of polycarbonate. Other compounds known to lower the viscosity of resins cause degradation of polycarbonate resins. Some compounds, conventionally employed to improve the workability of polymers, produce an embrittling effect on polycarbonates when they are mixed therewith and the resin is subjected to elevated temperatures as in molding. Still other materials, while satisfactory stiffness modifying agents for other plastics, are too volatile to be incorporated with polycarbonates since polycarbonates have much higher melting points than many other thermoplastics. Furthermore, aromatic polycarbonates, especially in the form of pigmented compositions, e.g., containing TiO.sub.2 and the like, are thermally unstable at high molding temperatures and tend to become yellow. It has been proposed to overcome these problems by adding polydiorganosiloxanes, see Caird, U.S. Pat. No. 3,087,908, or tiorganophosphites, e.g., U.S. Pat. No. 3,305,520, or mixtures of phosphonites and an epoxy compound, e.g., U.S. Pat. No. 3,729,440, or a phosphite, epoxy compound, ultramarine pigment and aryl siloxane fluid, see, e.g., U.S. Pat. No. 3,673,146, the disclosures of which are incorporated herein by reference.
It has now been surprisingly discovered that, by admixing a minor amount of a siloxane-oxyalkylene block copolymer with an aromatic carbonate polymer, and, optionally, an organic phosphite or phosphonite and/or epoxy compound, the resultant polycarbonate composition has reduced melt viscosity and does not become brittle or degraded upon molding and thus retains its characteristic high impact strength.